Fix unsupported linker flag on Mac (#4473)

Summary:
Run into the following linker error on Mac:
```
[ 83%] Linking CXX executable llama_main
ld: warning: -s is obsolete
ld: unknown options: --gc-sections
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [llama_main] Error 1
make[1]: *** [CMakeFiles/llama_main.dir/all] Error 2
make: *** [all] Error 2
```
Env:
`clang --version`
```
Apple clang version 15.0.0 (clang-1500.3.9.4)
Target: arm64-apple-darwin23.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
```

Fix:
It turns out that Apple Clang linker doesn't support the `--gc-sections` options, where it instructs the linker to remove unused sections from the final executable or shared library. Use an equivalent flags `-dead_strip` instead on Mac.

Pull Request resolved: https://github.com/pytorch/executorch/pull/4473

Reviewed By: cccclai

Differential Revision: D60474232

Pulled By: guangy10

fbshipit-source-id: d1f2745913a49588bdac4e8f5f792dfae4d7c9cd